Bulk elasticity of concentrated protein-stabilized emulsions

We prepared concentrated quasi-monodisperse hexadecane-in-water emulsions stabilized by various proteins, and we investigated their theological properties. Some protein-stabilized emulsions possess remarkably high elasticity, being at the same time considerably fragile-they exhibit coalescence at yield strain and practically do not flow. The elastic storage modulus G ' and the loss modulus G " of the emulsions were determined for different oil volume fractions above the random close packing. The results for G ' were normalized by the capillary pressure of the nondeformed droplets and further compared with literature data obtained on emulsions stabilized by classical surfactants. Surprisingly, the dimensionless elastic modulus G '/(sigma /a) (sigma being the interfacial tension and a being the drop radius) obtained for emulsions stabilized by different proteins does not collapse on a single master curve, being almost always substantially higher than the corresponding values obtained for equivalent surfactant-stabilized emulsions. Furthermore, we found G '/(sigma /a) to be correlated (for a given oil volume fraction) to the dilatational elastic modulus E of the protein layer adsorbed on the droplets. The osmotic equation of state of our emulsions was determined experimentally and was found to be almost identical with the one obtained for samples stabilized by classical surfactants. This provided a hint that the emulsions' elasticity is not connected with any specific properties of the bulk continuous phase. We were also able to produce elastic biliquid foams, containing practically no water, by free evaporation of the aqueous phase. This was possible most probably because of the formation of a relatively thick and rigid protein layer(s) that protects the fat globules against coalescence.